1. Field of the Invention
The present invention relates to a voltage generating circuit, and more particularly to a gamma reference voltages generating circuit.
2. Description of Related Art
Currently, a large number of products in the market for displaying images such as Thin Film Transistor Liquid Crystal Display (TFT-LCD) or Liquid Crystal on Silicon Liquid Crystal Display (LCoS-LCD) frequently use a gamma curve to calibrate the quality of display image. For example, when a liquid crystal display needs to display an image signal, a driving voltage corresponding to the image signal must be applied to drive the liquid crystals so as to rotate the liquid crystals to a definite angle. However, the relationship between the magnitude of the driving voltage and the effect of the subsequent rotation of the liquid crystals on human eye perception is non-linear. Therefore, in order for the human eyes to receive information produced by an image signal, a gamma curve designed to adjust the relation between the image signal and the driving voltage is required.
More specifically, the gamma curve modifies the characteristic conversion ratio curve of a liquid crystal material so that the human eye can identify the level of brightness of a display panel. To display high quality images, the gamma curve calibration can aim for a higher contrast and a higher gray scale resolution. In general, different gamma curves can strengthen and express the characteristic quality of the image so as to optimize the visual effect of the display.
FIG. 1 is a diagram of a conventional gamma reference voltages generating circuit. As shown in FIG. 1, a conventional gamma reference voltages generating circuit comprises a resistor string (RS), 19 10-bit digital-to-analog converters (DAC) 110˜128, 19 output buffers 130˜148 and a digital circuit control interface (such as an I2C interface) 150. FIG. 2 is a schematic three-dimensional view of the circuit in FIG. 1. In FIG. 2, the digital-to-analog converters and the output buffers are drawn in three dimensions so as to highlight their numbers.
As shown in FIGS. 1 and 2, each of the 10-bit digital-to-analog converters 110˜128 comprises a 9-bit PMOS digital-to-analog converter and a 9-bit NMOS digital-to-analog converter. Therefore, the present circuit requires a total of 19*2=38 9-bit digital-to-analog converters.
The gamma reference voltages comprise a first gamma reference voltage Vcom, second gamma reference voltages GM1˜GM9 and third gamma reference voltages GM10˜GM18. The steps for producing the gamma reference voltages are as follows. The divided voltages provided by the resistor string RS are output to the 10-bit digital-to-analog converters 110˜128 through conductive wires H0_0˜H1023_0. The digital circuit control interface 150 provides control signals to the 10-bit digital-to-analog converters 110˜128. According to the 1024 levels between the first source voltage VH to the second source voltage VL, each of the 10-bit digital-to-analog converters 110˜128 decodes to produce the first gamma reference voltage Vcom, the second gamma reference voltages GM1˜GM9 and the third gamma reference voltages GM10˜GM18 respectively. The decoded voltages are input to the output buffers 130˜148 through the conductive wires 160_1˜160_19 so as to output the gamma reference voltages.
In the conventional technique, each gamma reference voltage is generated by the same mechanism so that a lot of useful layout area is wasted.